Atmospheric Research Pty Ltd offer medium frequency SODARs based on research and development initially undertaken at the Point Cook Air Force Academy in Victoria, and subsequently the Australian Defence Force Academy in Canberra, by Dr Ian Bourne.
SODARS can be used for;
- Industrial process and thereby pollution control by real-time monitoring of the local atmospheric environment;
- Characterisation of the local micro-climate;
- Wind monitoring at airports and parachute drop zones;
- Wind Power assessment, including averages, distributions and wind shear data;
- Blast Noise Monitoring, Prediction and Minimisation.
What is a SODAR?
A SODAR (SOund Detection And Ranging) system is an instrument for the measurement of wind velocity, remotely, from the ground. It operates by transmiting a short pulse of sound which is refracted by the small scale turbulence in the atmosphere. This turbulence is transported by the wind, and the radial velocity of the air can be determined by measuring the Doppler shift of the sound being refracted from the turbulence. The range of the turbulence is determined from the delay between the transmission of the acoustic pulse, and the reception of the refracted signal. By repeating this process in three different directions, each direction having a large component being orthogonal to the other two directions, the three dimensional wind field can be calculated.
The SODAR sends out a sound pulse every 6 seconds in one of the three directions, switching sequentially between these directions. The signal processing algorithm includes extensive filtering and averaging, to ensure a good signal to noise measurement. The frequency of the sound pulse is chosen to provide a compromise between attenuation (which increases with frequency) and environmental noise. A frequency of 1875 Hz is used by the ARPL SODAR. The wind velocity (in X, Y, Z) versus altitude, is displayed on the display electronics approximately every 3 minutes. computer, and is written to file for long term data logging.
The SODAR consists of three main subsystems;
- Antenna Subsystem;
- Control Electronics Subsystem;
- Display Computer Subsystem.
A block diagram of the SODAR instrument is shown below. Note that the control and display computer functions can be carried out with one computer, or two (for remote operation).
As well as three-axis SODARs, two axis units can be used where only horizontal wind data is required. Alternatively a phased array configuration can be used, as shown below.
- Two-axis Antenna option. Two axis units can be used where only horizontal wind data is required. This allows more rapid sampling of the horizontal wind components, for higher signal to noise.
Phased Array Antenna Option. As an option, the three fixed antennae may be replaced by a phased antenna array. This provides a compact format, which may be advantageous where space is limited. Phased array systems have the following disadvantages;
- More complex electronics and software, and thus a higher price;
- More failure modes, due to greater complexity;
- Greater susceptibility to environmental noise, due to problems with sidebands from the antenna gain pattern.
- Simultaneous Sounding Option. As an option, the SODAR can be configured for simultaneous sounding on all three axes. This provides a three-fold increase in data rate, and hence an improvement in signal to noise and/or in temporal response.
- Ground Sensor Suite. A complete ground-level meteorological sensor suite (temperature, relative humidity, atmospheric pressure, wind velocity and direction, or a subset of these) can be provided, with data logging via the instrument computer.
- Integration of the SODAR and a RASS. The baseline SODAR is not supplied as an add-on to a RASS. This is not recommended and gives a performance compromise. If the customer needs a combined RASS/SODAR, however, a combined unit can be delivered.
- Containerised Configuration. The medium frequency SODAR can be supplied in a pair of standard shipping containers, to allow rapid installation, re-location and removal, and simplified site preparation. A typical 2-axis installation with solar-battery operation is shown below.
- Modifications to the configuration of the hardware, or to the software (eg to the user display), may be undertaken by ARPL, as options within a contract, or as subsequent upgrade contracts.
2-Axis Medium Frequency SODAR in shipping containers, with SOLAR-battery power.
Advantages of SODARS?
- Continuous operation and measurement;
- No intrusion into the space being sampled;
- Low labour costs for measurements;
The state-of-the-art performances of the ARPL SODAR are summarised in the table.
Limitations of SODAR Operation
The limitation to a SODAR’s operation stem principally from the use of acoustics as a probing technique.
- Range. Sound is attenuated in the atmosphere, with higher frequency sound attenuated much more than lower frequencies. With increasing temperatures, and/or lower relative humidity, the attenuation of sound increases. The height performance of a SODAR in a hot, dry, desert may only be 60% of the same instrument in a cool, damp, location.
- Audible Sound. The use of sound can limit its use in built-up areas.
- Background Noise. The background noise where a SODAR is operating can also limit aa SODAR’s performance. In general SODARs should not be operated in areas where the noise level (at the frequency of operation of the SODAR) is high.
- Local Structures. SODARs should not be installed near structures (or vegetation) which can produce fixed echoes.
The following plots show the correlation between the SODAR measurements of wind direction, and those measured at 50m and 75m on a nearby instrumented tower. They show very good agreement between the instruments.
|Horizontal Wind Speed Components||Range 0-20 m/s, accuracy 0.2 m/s|
|Horizontal Wind Speed Vectors||Range 0 - 25 m/s|
|Vertical Wind Speed Components||Range 0 - 10 m/sec, accuracy 0.1 m/s|
|Horizontal Wind Direction||0 - 359 degrees|
|Resolution of Reading||0.1 m/s|
|Sampling Height||50 m - 900 m AGL|
|Data Interval||Approximately 10 minutes|
|Display Latency||Real Time|
|Environmental Conditions||-10C to + 40C, 0 - 100% humidity|
|Power Supply||240V (others available)|
|Acoustic Frequency||1525 - 2225 Hz (selectable)|
|Configuration Options||Two or three axis. Slab mounted or built into shipping container.|
SODAR Control Unit, showing industrial computer (right), processing electronics boards (top right), interface, power amplifier (top left) and audio electronics (bottom left). Unit can run on 240V, 24V Batteries (with solar array) or portable generator.